1. Field of the Invention
The present invention relates to an electric car and, more particularly, to a motor control system for the electric car.
2. Description of the Related Art
In one prior art electric car, a battery is mounted in the car body as a power source of DC current which is converted into three-phase AC current to drive a motor. The speed of the car is changed by controlling the torque of the motor which is connected to the drive wheels of the car. The motor control unit includes an electronic control unit (ECU) for generating pulse-width modulated (PWM) base drive signals corresponding to the load conditions of the electric car. The PWM base drive signals are applied as switching signals to an inverter bridge.
In the inverter bridge, the PWM base drive signals switch IGBTs and power transistors operating as current control elements to convert the DC current of the battery into three-phase AC currents having U, V and W phases which are fed to the coils of the motor. Typically, the inverter bridge has six IGBTs divided into three pairs or arms which generate the currents in the respective individual phases. One IGBT of each pair is turned ON by pulses which have pulse widths designed to generate a corresponding half-cycle of each phase current while the other IGBT of each pair is held OFF until the succeeding half-cycle when operation of the IGBTs in each pair is reversed. Base drive signal amplifiers are provided for amplifying the PWM base drive signals generated by the ECU and for applying the amplified drive signals to the bases of the IGBTs.
The DC current of the battery, which has a voltage of 12 volts, is input to a base drive power circuit (e.g., DC-DC converter). The DC=DC converter transforms the 12 volt input into an output voltage of 30 volts which is fed to the base drive signal amplifiers.
In the prior art motor control system, the output voltage of the base drive power circuit may drop to disable the turn OFF of one IGBT. If the corresponding other IGBT is then turned ON, the positive and negative sides of the arm are shorted to burn out the paired IGBTs. Still worse, a large reverse current flows through the bases of the two IGBTs to burn out the base drive power circuit.
Additionally, the transformer of the base drive power circuit has power elements such as FETs on its primary side. If the input voltage of the base drive power circuit drops, the current flow into the power elements increases at an abnormal rate to maintain the output voltage. This current flow increases burn out of the base drive power circuit.